In dyeing textile fabric, it is often desirable to remove the color from the dyed fabric in order to correct faulty dyeing or to redye surplus fabric to a different color for reuse. Stripping is the process which is used to remove dye from dyed fabrics, and the process is either designated "back stripping" or "destructive stripping". In back stripping, only the depth of shade is changed while in destructive stripping, the dye is chemically altered. For example, dyes containing an azo group (--N.dbd.N--) can be chemically reduced to an almost colorless amine compound by using chemical reducing agents.
It is important that the stripping process, whether back or destructive stripping, effectively and efficiently remove the dye without damping the fabric and without leaving a residue which would affect subsequent redyeing of the fabric. Textile fabrics are commonly stripped using various stripping liquids. Stripping liquids usually contain a combination of a chemical reducing agent and a stripping assistant. Stripping assistants are chemicals for improving the stripping ability of reducing agent. Some of the stripping assistants known to be used in prior art stripping liquids are quaternary ammonium salts, formic acids, and benzyl alcohol. For example see "Stripping of Dyestuffs From Textiles", by R. B. Chavan Silk and Rayon Industries of India, February, 1969, pp. 167--170, and U.S. Pat. No. 3,591,325 to Sapers both of which are incorporated herein by reference.
In the past, various chemical combinations of reducing agents and stripping assistants have been used in stripping liquids in order to strip dyes from textile fabrics. Direct dyes are stripped by boiling the fabric in alkaline sodium hydrosulfite or by bleaching the fabric with sodium hypochlorite or by boiling the fabric with 1-2% sodium chlorite adjusted with formic or acetic acid to a pH of 3 to 4. Vat dyes are stripped by treating fabric in a heated reducing bath containing sodium hydroxide, sodium dithionite and a substance which acts to combine with the dye in order that it does not recombine with the textile material once it is reduced. Vat dyes are also stripped from fabric by treatment of fabric with a stripping liquid containing caustic soda, sodium hydrosulfite and a quaternary ammonium salt. Sulphur dyes are stripped from fabrics with hot solutions of soda ash and sodium sulphide. Dispersed dyes can be removed from fabric with hot solutions of sodium hydrosulfite and ammonium hydroxide or zinc sulphoxylate formaldehyde and formic acid or sodium chlorite and formic acid. Most dyes can be stripped from polyester fabric with sodium hydroxymethane sulfinate in the presence of a carrier and acetic acid. These methods of stripping dyed fabric are disclosed in "Stripping of Dyestuffs from Textiles", by R. B. Chavan, Silk and Rayon Industries of India, February, 1969, pp. 167-170, and U.S. Pat. No. 3,591,325 to Sapers, both of which are incorporated herein by reference.
None of the various stripping liquids described in the prior art has had universal acceptance in the fabric dyeing industry. None of the stripping liquids could be used on a variety of fabrics and dyes; rather the stripping liquids had to be individually designed depending on the fabric and the dye being removed. In addition, many of the prior art stripping liquids had a pH greater than 9 which could have a harmful affect on some fabrics. These strong alkaline stripping liquids were used in order to break the bond of the dye to the fabric, but often left a stripped fabric which was difficult to redye successfully.